Interneurons in the ventral half of the spinal cord play important roles in the central control of movement. Many of these neuronal cell types arise from the four early ventral interneuron (IN) subtypes. The generation of mice in which all four ventral interneurons populations have been selectively marked creates a unique opportunity to identify and analyze neuronal connections in the spinal cord. Using genetically marked mice, it has been found that the most ventrally arising INs (V3 INs) appear to provide glutamatergic input to limb muscle motor pools and Renshaw cells (RCs) that mediate recurrent inhibition of motor neurons. It is hypothesized that V3 INs are essential components of the locomotor central pattern generator (CPG) and that the glutamatergic drive into RCs may play an important role in motor coordination. The experiments outlined in this proposal will assess the contribution of V3 INs to the mammalian locomotor CPG. These studies will also examine the nature and function of these putative excitatory inputs on Renshaw cells and ascertain the role of these connections during locomotion. These analyses will lead to a greater understanding of the spinal mechanisms controlling movement in walking animals.